Heretofore, non-contact type tonometers of the type disclosed in U.S. Pat. No. 3756073 have been known. In this conventional non-contact type tonometer, fluid for determining a cornea is discharged along an optical axis of an alignment optical system for observing an eye under test, toward the cornea to deform the cornea. The intraocular pressure is measured based on the deformation of the cornea. If the working distance from a discharging end of an orifice to the cornea is incorrect, and the vertex of the cornea is not in alignment with the optical axis of the alignment optical system, errors are produced in the intraocular pressure measurement. Therefore, a non-contact type tonometer of this type is provided with an alignment verification apparatus.
FIG. 8 is a schematic illustration of an alignment verification optical system functioning as the aforementioned alignment verification apparatus. In FIG. 8, 1 denotes a cornea of an eye under test. The alignment verification optical system includes an objective lens 2, a half mirror 3, a reticle plate 4. The objective lens 2 is used for the alignment optical system and disposed opposite a front part of the cornea 1. The objective lens 2 is provided with an orifice tube 5 penetrating therethrough. A target ray is projected on cornea 1 by a projection optical system (not shown). The target ray reflected from the cornea 1 is guided to the objective lens 2 as a corneal specular reflection ray and forms a target image on the reticle plate 4 by the objective lens 2.
A specialist observes the target image formed on the reticle plate 4 and effects the alignment verification in such a manner that the focussing position of the objective lens 2 will be brought to alignment with the center of curvature 1a of the cornea 1. The optical axis of the objective lens 2 will pass the center of curvature 1a of the cornea 1 through the vertex 0 of the cornea 2. Due to the alignment verification, the target image is clearly formed on the reticle plate 4. A part of the corneal specular reflection ray is reflected by the half mirror 3 and guided to a light detector 6. Accordingly, the non-contact type tonometer is capable of measuring the intraocular pressure based on detection signal of the light detector 6. That is, a fluid actuation device is actuated based on the detection signal, fluid is discharged from the orifice tube 5, the cornea 1 is deformed from convexity, through applanation, to concavity, and the intraocular pressure is measured based on the measured deformation.
The conventional non-contact type tonometer, as shown in U.S. Pat. No. 3756073 is constructed such that when an adjustment or verification is required with respect to the distance (working distance) between the cornea 1 of the eye under test and the orifice tube 5, the alignment verification is effected by bringing the focussing position of the objective lens 2 into alignment with the of curvature center 1a of the cornea 1. Accordingly, if the radius of the curvature of the cornea 1 is not constant, the alignment verification is effected such that the working distance is fluctuated to the extent of the difference in the radius of curvature across the cornea, i.e., (S.sub.1 -S) as shown in FIG. 8. Accordingly, in the conventional non-contact type tonometer, the errors based on the radius of the curvature of the cornea 1 is directly proportional to errors of the alignment verification. This means that a unique error is produced in measurement of intraocular pressure of an eye under test depending on the shape of a particular patient's cornea. Thus, the conventional non-contact type tonometer lacks accuracy of measurement.
Furthermore, since the alignment verification is effected based solely on clarity of the target image, there is involved the problem that it is not easy to confirm whether the target image is clearly formed, and the alignment verification can not be effected promptly.